1. Field of the Invention
The present invention relates to a lithographic printing plate precursor and a lithographic printing plate method using the same. More specifically, the present invention relates to a lithographic printing plate precursor capable of directly producing a printing plate by scanning an infrared laser based on digital signals of a computer or the like, so-called direct plate-making, and a lithographic printing method of performing printing by developing this lithographic printing plate precursor on a printing press.
2. Description of the Related Art
The lithographic printing plate in general consists of a lipophilic image area of accepting an ink in the printing process and a hydrophilic non-image area of accepting a fountain solution. The lithographic printing method is a printing method utilizing the repellency between water and oily ink from each other, where the lipophilic image area of the lithographic printing plate and the hydrophilic non-image area are formed as an ink-receiving part and a fountain solution-receiving part (ink non-receiving part), respectively, to cause difference in the ink adhesion on the surface of the lithographic printing plate, an ink is attached only to the image area and thereafter, the ink is transferred to a material on which an image is printed, thereby performing printing.
For producing this lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon a photo-sensitive resin layer (image-recording layer) has been heretofore widely used. Usually, a lithographic printing plate is obtained by a plate-making method where the lithographic printing plate precursor is exposed through an original image such as lith film and while leaving the image-recording layer in the image area, the image-recording layer in the non-image area is dissolved and removed with an alkaline developer or an organic solvent to expose the hydrophilic support to the surface.
In the plate-making process using a conventional lithographic printing plate precursor, a step of dissolving and removing the non-image area with a developer or the like according to the image-recording layer must be provided after exposure and as one problem to be solved, it is demanded to dispense with or simplify such an additive wet processing. In particular, the treatment of waste solutions discharged accompanying the wet processing is recently a great concern to the entire industry in view of consideration for global environment and the demand for solving the above-described problem is becoming stronger.
As one of simple plate-making methods, a method called on-press development has been proposed, where a recording layer enabling the removal of the non-image area of a lithographic printing plate precursor in a normal printing process is used and after exposure, the non-image area is removed on a printing press to obtain a lithographic printing plate.
The on-press development method specifically includes, for example, a method using a lithographic printing plate precursor having an image-recording layer dissolvable or dispersible in a fountain solution, an ink solvent or an emulsified product of fountain solution and ink, a method of mechanically removing the image-recording layer by the contact with an impression or blanket cylinder of a printing press, and a method of weakening the cohesion of the image-recording layer or adhesion between the image-recording layer and the support by the impregnation of a fountain solution, an ink solvent or the like and then mechanically removing the image-recording layer by the contact with an impression or blanket cylinder.
In the present invention, unless otherwise indicated, the “development processing step” indicates a step where by using an apparatus (usually an automatic developing machine) except for a printing press, the unexposed portion of the image-recording layer of a lithographic printing plate precursor is removed through the contact with a liquid (usually an alkaline developer) to expose the hydrophilic support surface, and the “on-press development” indicates a method or step where by using a printing press, the unexposed portion of the image-recording layer of a lithographic printing plate precursor is removed through the contact with a liquid (usually a printing ink and/or a fountain solution) to expose the hydrophilic support surface.
On the other hand, a digitization technique of electronically processing, storing and outputting image information by using a computer has been recently widespread and various new image-outputting systems coping with such a digitization technique have been put into practical use. Accompanying this, a computer-to-plate (CTP) technique is attracting attention, where digitized image information is carried on a highly converging radiant ray such as laser ray and a lithographic printing plate precursor is scan-exposed by this ray with no intervention of a lith film to directly produce a lithographic printing plate. Accordingly, one of important technical problems to be solved is to obtain a lithographic printing plate precursor suitable for such a technique.
As described above, the demand for a simplified, dry-system and non-processing plate-making work is ever-stronger in recent years from both aspects of consideration for global environment and adaptation for digitization.
However, when conventional image-recording systems using light in the region from ultraviolet to visible light are used for simplified plate-making work such as on-press development, the image-recording layer is not fixed even after exposure and the lithographic printing plate precursor possesses sensitivity to room light and therefore, must be kept in a completely light-shielded state after unpackaging until completion of the on-press development.
Recently, high-output lasers such as YAG laser and semiconductor laser of radiating an infrared ray at a wavelength of 760 to 1,200 nm are inexpensively available and a plate-making method using these high-output lasers for the light source on recording an image is promising as a method of producing a lithographic printing plate by scanning exposure which is easy to incorporate into a digitization technique.
In conventional plate-making methods using light in the region from ultraviolet to visible light, the image recording is performed by imagewise exposing a photosensitive lithographic printing plate precursor with low to middle intensity illuminance to cause a photochemical reaction in the image-recording layer and thereby imagewise change the physical properties. On the other hand, in the above-described method using a high-output laser, a large quantity of light energy is irradiated on the exposure region within an extremely short time to efficiently convert the light energy into heat energy and due to this heat, the image-recording layer undergoes chemical change, phase change or thermal change such as change of form or structure, which change is utilized in the image recording. Accordingly, the image information is inputted by light energy such as laser light, but the image recording is performed through a reaction by heat energy in addition to light energy. This recording system making use of heat generation by high-power density exposure is generally called heat-mode recording and the conversion of light energy into heat energy is called light-to-heat conversion.
The plate-making method using heat-mode recording is greatly advantageous in that the image-recording layer is not sensitive to light of normal illuminance level, such as room illumination, and fixing is not essential to the image recorded by high-intensity exposure. That is, the lithographic printing plate precursor for use in the heat-mode recording is safe to room light before exposure and fixing of the image after exposure is not essential. Accordingly, for example, when an image-recording layer which can be insolubilized or solubilized by exposure using a high-output laser is used and a plate-making process of imagewise removing the exposed image-recording layer to obtain a printing plate is performed by on-press development, this is expected to realize a printing system of causing no effect on an image even if the image is subjected to ambient light in a room after exposure. The realization thereof is demanded.
As such a lithographic printing plate precursor, for example, a lithographic printing plate precursor where an image-forming layer comprising a hydrophilic binder having dispersed therein hydrophobic thermoplastic polymer particles is provided on a hydrophilic support is known (see, for example, Japanese Patent No. 2,938,397). This lithographic printing plate precursor can be exposed by an infrared laser to cause coalescence of hydrophobic thermoplastic polymer particles due to heat and thereby form an image, then loaded on a cylinder of a printing press, and on-press developed by supplying a fountain solution and/or an ink.
However, in such a method of forming an image through coalescence by mere heat fusion of fine particles, the image strength is extremely low and the press life is not satisfied, despite good on-press developability.
As a technique of improving the press life, an on-press developable lithographic printing plate precursor comprising a hydrophilic support having thereon an image-recording layer containing a polymerizable compound-enclosing microcapsule is known (see, JP-A-2001-277740 and JP-A-2001-277742 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)).
Also, an on-press developable lithographic printing plate precursor comprising a support having provided thereon a photosensitive layer containing an infrared absorbent, a radical polymerization initiator and a polymerizable compound is known (see, JP-A-2002-287334).
In these methods utilizing a polymerization reaction, the image density can be enhanced by virtue of high chemical bond density in the image area as compared with the image area formed by the heat fusion of polymer fine particles, but in view of practical use, the on-press developability and the press life are still not satisfied.